(1) Field of the invention
The present invention generally relates to a method for depositing a thin film on a substrate by a sputtering process, and more particularly to a method for depositing a thin film on a substrate by a sputtering process in which a high quality thin film can be stably deposited on the substrate at a high sputtering rate.
(2) Description of related Art
In semiconductor devices, magnetic devices, optical devices, and the like, each of which has a thin film formed of nonmagnetic metal or magnetic metal, the thin film is deposited on a substrate by a sputtering process. The characteristics of the thin film deposited on the substrate depend on the ambient conditions under which the sputtering is performed. Thus, the thin film is deposited on the substrate under conditions in which residual gases, such as water vapor, which cause the quality of the thin film to deteriorate, are completely removed from the chamber in which the sputtering process is performed.
Recently, it has been required that thin films whose quality is as high as possible, be formed on the substrate. To achieve the requirement in the sputtering process which is performed in a chamber in which an inert gas such as argon is filled, the amount of residual of residual gas must be decreased and the thin film must be deposited on the substrate at a high sputtering rate so that the sputtering process can be completed in a short time. In another type of sputtering process in which a thin film formed of a nitride or an oxide is deposited on the substrate, a reactive gas such as nitrogen or oxygen is supplied with the inert gas to the chamber in which the sputtering process is performed. In this case, to obtain a high quality thin film, the amount of the reactive gas supplied to the chamber must be determined. This is, to deposit the thin film on the substrate at a high sputtering rate, an amount of the reactive gas which is proportional to the sputtering rate must be supplied to the chamber. As the discharge voltage in the reactive gas is higher than that in an inert gas such as argon, when the amount of the reactive gas in the chamber is too large, it is difficult for the discharge to be normally performed in the chamber. As used herein, the discharge voltage means the voltage at which the flow discharge starts. Thus, the amount of reactive gas supplied to the chamber, in which the thin film is deposited on the substrate by the sputtering process, must be optimally controlled.
In a conventional method for depositing a thin film on a substrate by a sputtering process, a sputtering device shown, for example, in FIG. 1 is used. Referring to FIG. 1, a target 30 is mounted on a cathode assembly 3 by screws 17a and 17b. The target 30 is formed of a backing plate 33 and a target plate 31 to be sputtered. The target plate 31 is adhered to the backing plate 33 by a solder 34 of metal having a low melting point. The backing plate 33 is made, for example, of copper or copper alloy, and the solder 34 is made of a material having a melting point of approximately 200.degree. C., such as indium alloy or tin alloy. This melting point of the solder 34 is less than that of the target plate 31. The cathode assembly 3 has a support frame 13b having a flange 3a which is formed on a lower end of the support frame 3b. Magnets, electromagnets 5 and other parts of the sputtering device are mounted in the support frame 3b. A flow path 9 is formed around the magnets, the electromagnets 5 and the like in the support frame 3b. A heat exchanging medium 2a, such as water, is supplied via an inlet 6a to the flow path 9. The heat exchanging medium 2a flows through the flow path 9 and is withdrawn from an outlet 6b. The target 30 is mounted on the cathode assembly 3 via a sealing member 4 (an O-ring) so that the rear surface of the backing plate 33 is in contact with the heat exchanging medium 2a. A chamber 10 in which a substrate is placed is provided so as to be connected to the support frame 3b.
In the above sputtering device, to prevent the solder 34 from melting, the target plate 31 is cooled by the heat exchanging medium 2a (e.g. water) via the backing plate 33 while the sputtering deposition is being performed. Thus, the target plate 31 is sputtered at a relatively lower temperature, such as a temperature less than approximately 150.degree. C.
While the target plate 31 is being sputtered in the chamber 10 which is filled with the argon gas, the thin film, made of atoms of the target plate 31, is being deposited on the surface of the substrate which is placed so as to face the target plate 31. If a reactive gas, such as nitrogen or oxygen, is in the chamber 10, the sputtering is performed under a condition in which absorption of the reactive gas occurs on the surface of the target plate 31. In this case, a nitride thin film or an oxide thin film is deposited on the surface of the substrate.
FIG. 2 shows an example of target in which solder, a brazing filler metal having a low melting point is not used. Referring to FIG. 2, the target plate 31 is directly mounted on the flange 3a of the support frame 3b by the respective screws 17a and 17 b via the sealing member 4 (the O-ring). Thus, the target plate 31 is directly cooled by cooling water (the heat exchanging medium).
In the case where the thin film is deposited on the substrate by the sputtering device shown in FIG. 1, the target plate 31 is sputtered at a relatively low temperature, as has been described above. Thus, there may be a case where the sputtering starts under a condition where physical absorption of residual gases such as water vapor occurs on the surface of the target plate. In this case, as the water vapor is diffused from the target out into the chamber when the sputtering is performed, the quality of the thin film deposited on the substrate deteriorates.
In addition, if the sputtering is performed under conditions where a reactive gas is in the chamber 10, the following disadvantage occurs. If the target plate 31 has a low temperature, the partial pressure of the reactive gas in the chamber 10 must be increased to perform the sputtering at a high sputtering rate. However, when the amount of the reactive gas in the chamber 10 is increased so that the partial pressure of the reactive gas is increased, it becomes difficult for the discharge to occur in the chamber 10. Therefore, when the sputtering is performed at a low temperature, it is difficult for the thin film of the nitride or the oxide to be stably deposited on the substrate at a high sputtering rate.
In a case where the thin film is deposited on the substrate by sputtering in which the target shown in FIG. 2 is used, as the target plate 31 is directly mounted on the cathode assembly 3, the power which can be supplied to the target plate 31 for the sputtering depends on the thermal conductivity of the target plate 31 itself. That is, the greater the thermal conductivity of the target plate 31, the larger the amount of power which can be supplied to the target plate 31. Thus, when a target plate 31 having a low thermal conductivity, such as a silicon plate, is used for the sputtering, the power which is supplied from the cathode assembly 3 to the target plate 31 must be limited. In this case, it is difficult for the sputtering to be performed at a high sputtering rate.